if you note that someone does download the manuals, it may be good opportunity to write something as "Noticed you joined the forum to obtain the manuals if you're having any problems with your vehicle, or would like more information on such awesome reliable vehicle, myself and many others on this forum would be the happy to help with advice etc to resolve any issues". that would be a far more encouraging way to maintain members and there commitment to the forum.

The manuals are a golden opportunity to gain members, get to know the potential member and establish a great working relationship.